1. Field of the Invention
The present invention relates to a noncontact IC card, a card reader/writer, and a card system, which employ radio waves as communication media thereof.
2. Description of the Related Art
FIG. 24 shows a card apparatus (card system), which is disclosed in AIM JAPAN [Data carrier Technic and Application] 1990 Nikkan Kogyo Shinbun. This card system includes a conventional card reader/writer 1 and a noncontact IC card 2. FIG. 25 shows a receive circuit 17 of the conventional card reader/writer 1.
In FIG. 24, the card reader/writer 1 includes an input/output circuit 11, which is connected to a host computer (HOST) to communicate with the host computer, a control unit 12, which controls the whole system and processes data, a UART (universal asynchronous receiver transmitter) 13, which converts serial data to parallel data and vice versa, a send circuit 14, which modulates the digital signals of serial data received from the UART 13 and converts them to electrical signals, a transmitting antenna 15, which includes a parallel resonance circuit and transmits the electrical signals as radio waves, a receiving antenna 16, which includes a parallel resonance circuit and converts received radio waves into electrical signals, a receive circuit 17, which demodulates the electrical signals and converts then into digital signals, and a power circuit 18, which supplies a power source of, for example, 5 VDC, to the foregoing constituents and a power source of, for example, 24 VDC, to the transmitting antenna 15 and the receiving antenna 16. The control unit 12 includes a CPU, ROM, RAM, etc.
Further in FIG. 24, the noncontact IC card 2 is equipped with a transmit-receive antenna 21, which converts received radio waves into electrical signals and vice versa, a receive circuit 22, which demodulates the electrical signals and converts them into digital signals, a UART (universal asynchronous receiver transmitter) 23, which converts serial data to parallel data and vice versa, a control unit 24, which controls the whole system and processes data, a send circuit 25, which modulates the digital signals of the serial data received from the UART 23 and converts them into electrical signals, and a power circuit 26, which includes a battery. The control unit 24 is equipped with a CPU, ROM, RAM, etc.
In FIG. 25, the receive circuit 17 is provided with a voltage controlled receiving buffer amplifier 171 connected to the receiving antenna 16, an AGC (automatic gain control) 172, which automatically controls the gain of the voltage controlled receiving buffer amplifier 171, and a demodulator 173 connected between the receiving buffer 171 and the UART 13.
The operation of the conventional card reader/writer 1 and the noncontact IC card 2 will now be described with reference to FIGS. 26, 27, 28, and 29. FIGS. 26 and 27 show the forms of the signals sent from the card reader/writer 1 to the noncontact IC card 2. FIG. 28 shows a flowchart which gives a brief overview of the operation of the noncontact IC card 2. FIG. 29 shows a flowchart which gives the details of the operation corresponding to steps 33 and 34 in FIG. 28.
As a basic operation, the card reader/writer 1 sends a command to the noncontact IC card 2 and the noncontact IC card 2 executes the command and sends the result back to the card reader/writer 1. For instance, when the system is applied to a security system, the card reader/writer 1 is installed on a door of a room, while the noncontact IC card 2 is carried by an inhabitant of the room. The card reader/writer 1 continually sends a command, requesting an ID to be returned, and when the noncontact IC card 2 receives the command, it returns the ID. The card reader/writer 1 checks the ID and unlocks the door if it accepts the ID. There are some cases where the host computer connected to the card reader/writer 1 checks the ID.
As shown in (a) of FIG. 26, the signal sent from the card reader/writer 1 to the noncontact IC card 2 is configured so that a trigger signal is sent out first, then a start flag is sent out following a 2-bit interval, and the data shown in FIG. 27 follow.
A frame, which is a unit of data, includes a 1-bit start bit, 8-bit data, a 1-bit parity bit, and a 2-bit stop bit as shown in FIG. 27. The start flag of FIG. 26 indicates the data which have the same configuration as that of the single frame of FIG. 27, the 8-bit data in the single frame having a predetermined bit pattern.
First, upon receipt of the trigger signal shown in (a) of FIG. 26, the noncontact IC card 2 is switched from a sleep state to an active state and enables the UART 23 to receive signals (refer to steps 30 through 32 of FIG. 28). Then, when the noncontact IC card receives the start flag shown in (a) of FIG. 26, it begins receiving a data block shown in FIG. 27 (refer steps 33 and 34), implements the processing in response to the command, sends the result back to the card reader/writer 1 or carries out other appropriate processing, then moves back into the sleep state (refer to a step 35).
The UART 23 of the noncontact IC card 2 starts receiving the data when it detects the 1-bit start bit and receives the following 8 bits as data (refer to a step 40 of FIG. 29). The 2-bit stop bit indicates the break between frames; if this break cannot be detected, it is determined as a framing error and the data received this time is not taken in, and the system waits until it detects the next start bit (1 bit indicating the change from the high level to the low level as shown in FIG. 27), i.e., until synchronization is achieved before it goes back to the step 40 (refer to steps 41 and 42). If no framing error occurs, then the system accepts the next data (refer to a step 43).
In other words, if the start bit is not properly detected, it is very likely that the next start bit cannot be properly detected until the framing error takes place.
As shown in (a) of FIG. 26, after the trigger signal is received, the start flag, which is the data block having a particular bit pattern, is received. The noncontact IC card 2 enables the UART 23 to receive signals only after the end of the trigger signal; therefore, the start flag can be surely received if the trigger signal is properly received.
If, however, the trigger signal is left out as indicated by the dotted line shown in (b) of FIG. 26, resulting in a failure to properly receive the trigger signal (e.g., if the distance between the card reader/writer 1 and the noncontact IC card 2 is just barely within the communication range), then the noncontact IC card 2 is actuated to make the UART 23 ready for receiving, causing a portion of A to be erroneously received as the start bit in the start flag. The UART 23 processes data on an 8-bit basis; therefore, the following start flag cannot be properly received, skipping the whole data block.
The operation of the receive circuit 17 of the card reader/writer 1 will now be described.
The intensity of a radio wave greatly varies depending on the distance between the card reader/writer 1 and the noncontact IC card 2. For this reason, the gain of the receive circuit 17 needs to be adjusted. This is why the receive circuit incorporates an AGC 172. The AGC 172, however, requires some time to restore its original gain once the gain decreases. The card reader/writer 1 has the transmitting antenna 15 and the receiving antenna 16 which share the same frequency; therefore, it receives the radio waves (extremely intense), which the card reader/writer itself sends out through its own receiving antenna 16. This causes the gain of the AGC 172 to decrease for a while after transmission. As a result, the communication distance between the card reader/writer 1 and the noncontact IC card 2 is shortened.
This means that there was a problem with the conventional noncontact IC card 2 in that, if it fails to properly receive the trigger signal from the card reader/writer 1, then it cannot securely receive the start flag and skips the whole data block, resulting in an increased communication error rate with consequent damaged communication reliability.
There was also a problem with the conventional card reader/writer 1 in that, since it uses the same frequency for transmitting and receiving, it receives its own transmitted radio waves through the receiving antenna 16. These intense radio waves decrease the gain of the AGC 172 of the receive circuit 17, which in turn deteriorates the receiving sensitivity and shortens the communication distance. This led to a higher communication error rate with consequent deteriorated communication reliability.
There was still another problem with the conventional card reader/writer 1 in that, since it uses the same frequency for transmitting and receiving, the transmitting antenna 15 and the receiving antenna 16 thereof interfere with each other. This destroys the waveforms of transmitted and received signals, leading to a higher communication error rate and poor communication reliability.